That's completely up to you and your preferences/site.
If you're just using a forum with phpBB, you might want to use the phpbb focussed one.
However, if you also have an entire site around it, it's debatable.
Also if you've already submitted your sitemap for a while, I don't think it's smart changing it, but I am not an expert on sitemaps.

The CDB is Customization Database. A database of extensions for phpBB which have been validated by their Extensions team, which means the code is solid. Customization DatabasephpBB Extensions

Site maps are generally for sites with static data, to update it you need to edit it. For sites with dynamic data such as bulletin boards you need a dynamic site map and, I presume, the one James mentioned is dynamic to suit phpBB?

"The more connected we get the more alone we become" - Kyle Broflovski

It might be good to have a sitemap, but it is not NEEDED. Google is a lot cleverer than most people realise, and will index your forum without any problem.

As both Mick and I have stated, if you want a sitemap you the one I linked to previously is what you want to use. The static one you have already may be doing more harm than good. Delete it and use the one thats made for the purpose.

They are trying to help you.
You make a (rather unclear) enquiry and they provide you with an answer, in their spare time. So yeah, it might be possible that they don't instantly respond to a topic in General Discussion...

You have to download the SEO Sitemap Extension and place it in your Extensions folder as: root/ext/shredder/sitemap/readme.md
Then in your Administration Control Panel (ACP) under Customise enable the "phpBB3 SEO Sitemap" extension.